The invention relates to an electronic flash, and more particularly, to an electronic flash which permits rapid charging of a commutation capacitor, a continued emission of flashlight from a flash discharge tube for a given time interval in response to a single trigger operation, a rapid triggering operation or an interrupted series of emissions over a plurality of operations.
A conventional electronic flash, in particular, an electronic flash of series controlled type in which a switching element is connected in series with a flash discharge tube to provide an automatic emission control, includes a resistor having a high value connected in the charging path of the commutation capacitor, thereby preventing a discharge current from the flash discharge tube or an energizing current through a main thyristor, which serves as a switching element, from being bypassed through the charging path associated with the commutation capacitor. Accordingly, the magnitude of a charging current for the commutation capacitor is limited by the resistor, resulting in an inconvenience that an increased length of time is required to complete the charging of the commutation capacitor.
Specifically, FIG. 1 shows an exemplary electronic flash of the prior art which is chosen to illustrate the inconvenience mentioned above. Initially considering the arrangement of the electronic flash, it includes a power supply circuit 1 formed by a DC-DC converter which is known in itself. The positive terminal of the converter is connected to a positive bus l.sub.1 through a rectifier diode D1 while the negative terminal is connected to a negative bus l.sub.0. Connected across the buses l.sub.1 and l.sub.0 are a main capacitor C1; a series combination of a resistor R1 and a neon lamp Nel which indicates the completion of a charging operation; another series combination of a coil L1, a flash discharge tube FL1 and a main thyristor SR2; a further series combination of a resistor R3 and a commutating thyristor SR3; and an automatic emission control circuit EC1.
The anode of a thyristor SR1 is connected to the junction between the resistor R1 and the lamp Ne1 while the cathode is connected to the bus l.sub.0. The gate of the thyristor SR1 is connected to a trigger circuit TC1 which is in turn directly connected to the bus l.sub.0 and also connected to the bus l.sub.0 through synchro contacts SW0 of an associated camera. The anode of the thyristor SR1 is also connected to one end of a trigger capacitor C2, the other end of which is connected to the bus l.sub.0 through a primary coil of a trigger transformer T1. The secondary coil of the transformer T1 is connected to the bus l.sub.0 at its one end and connected to the trigger electrode of the flash discharge tube FL1 at its other end.
The coil L1 functions to provide a smooth transition for the leading and the trailing edge of the discharge current through the discharge tube FL1, and this coil is shunted by a diode D2. The main thyristor SR2 has its anode connected to the discharge tube FL1 and its cathode connected to the bus l.sub.0, while its gate is connected to the trigger circuit TC1. The anode of the thyristor SR2 is also connected to the bus l.sub.0 through a resistor R2 and also connected to one end of a commutation capacitor C3, the other end of which is connected to the junction between the resistor R3 and the commutating thyristor SR3. The thyristor SR3 has its anode connected to the resistor R3 while its cathode is connected to the bus l.sub.0. The gate of the thyristor SR3 is connected to the automatic emission control circuit EC1. A phototransistor PT1, which is provided for purpose of photometry, has its collector and emitter connected to the automatic emission control circuit EC1.
In operation, when the synchro contacts SW0 is turned on (i.e. closed), the trigger circuit TC1 operates to fire the both thyristors SR1 and SR2, thus initiating the emission of flashlight from the discharge tube FL1. Specifically, as the thyristor SR1 is fired, the trigger capacitor C2 is short-circuited therethrough, and the discharge of the capacitor C2 produces a current flow through the primary coil of the trigger transformer T1. This develops a high voltage across the secondary coil thereof, which is applied to the trigger electrode of the discharge tube FL1, thus exciting it. Accordingly, if the main thyristor SR2 is fired simultaneously, the main capacitor C1 discharges through a path including the coil L1, discharge tube FL1 and main thyristor SR2, whereby the discharge tube FL1 initiates its emission of flashlight.
When the phototransistor PT1 has received a proper amount of light after the initiation of the emission of flashlight from the discharge tube FL1, the automatic emission control circuit EC1 is activated and fires the commutating thyristor SR3. This causes the commutation capacitor C3 to discharge through the thyristor SR3, thus reversely biasing the main thyristor SR2, which is then turned off. The discharge current through the discharge tube FL1 then ceases, whereby the emission of flashlight terminates.
In the described arrangement, it is to be noted that the commutation capacitor C3 begins to be charged again through the resistors R3 and R2 when the current flow through the commutating thyristor SR3 reduces below a holding current level thereof to turn it off. However, the resistors R3 and R2 have such large values that the current flow through the discharge tube FL1 cannot be diverted through the resistor R2 when the thyristor SR2 is turned off and that the current flow through the resistor R3 and the thyristor SR3 is maintained below the holding current level of this thyristor when the commutating thyristor SR3 is turned on. For this reason, it takes a long time to charge the commutation capacitor C3 once the thyristor SR3 is turned off. If the commutating thyristor SR3 is re-fired before the charging of the commutation capacitor C3 is completed, there occurs no commutation, thus resulting in a failure to cease the emission of flashlight from the discharge tube FL1.
Representing the voltage to which the commutation capacitor C3 is charged by V.sub.3, we have EQU V.sub.3 =V.sub.1 ( 1-e.sup.-t/C 3.sup.(R 2.sup.+R 3.sup.)) (1)
where V.sub.1 represents the voltage to which the main capacitor C1 is charged, C.sub.3 the capacitance of the commutation capacitor C3, R.sub.2 and R.sub.3 the resistance of resistors R2 and R3, respectively, and t the time. Solving the equation (1) under the initial condition that V.sub.3 =0 at t =0, we have ##EQU1## where R=R.sub.2 +R.sub.3. Substituting values of C.sub.3 =2.2 .mu.F, R=40 k.OMEGA., V.sub.1 =300 V and V.sub.3 =250 V into the equation (2) yields a time length T.sub.0 required to charge the commutation capacitor C3 to 250 V as follows: ##EQU2## This means that a time interval on the order of at least 160 ms is necessary between successive commutations with a conventional electronic flash as mentioned above, considering time periods which are associated with the emission from the flash discharge tube FL1. In other words, such electronic flash will be limited to repeat its commutation in synchronism with a motor drive which is designed to take pictures at a rate of five frames per second.
As is well recognized, a photographic camera employing a focal plane shutter is subject to a disadvantage that a normal flash photography is disabled during a high speed shutter operation in which the electronic flash cannot be activated for emission in synchronism with the shutter operation. Thus, the focal plane shutter does not reach a full opening at a timing which is less than the synchronized timing of the electronic flash, while a slit defined between the first and the second blind runs in front of a film surface. In such instance, only part of the film surface is exposed to flashlight if the electronic flash is activated for emission at any time, thus preventing a uniform exposure.
To accommodate for such inconvenience, there has been proposed an electronic flash of the type which enables a continued emission substantially at a given level of flashlight during the time the slit runs in front of the film surface. Such electronic flash is disclosed in Japanese Laid-Open Patent Application No. 129,327/1980, for example. The electronic flash disclosed in this application basically comprises a series circuit formed by a flash discharge tube, a coil and a switching element connected across a main capacitor, and a diode connected in shunt with the series combination of the flash discharge tube and the coil. By turning the switching element on and off in an alternate fashion, power is derived intermittently from the main capacitor, and the time interval between the on/off condition of the switching element is controlled in accordance with a desired level of emission from the flash discharge tube so as to maintain an approximately constant emission level. During the time the switching element is on, a difference between the capacitor and the voltage across the discharge tube is applied to the coil which stores power in the form of a magnetic field, which is in turn returned to the discharge tube through the diode when the switching element is turned off, thus enabling a decayed emission from the discharge tube when the switching element is off.
However, in the described arrangement, the coil connected between the discharge tube and the switching element to limit the magnitude of the current flow acts to limit a discharge current through the discharge tube from the main capacitor, thus disadvantageously rendering it difficult to provide an emission of a uniform high level.
It should be understood that an electronic flash of the continued emission type which is available in the prior art is one which is devoted to the continued emission, and cannot also serve as an electronic flash of automatic emission control type.
Returning to the arrangement of FIG. 1, it will be noted that the trigger capacitor C2 begins to be charged through the resistor R1 again after the trigger thyristor SR1 is turned off. However, the value of the resistor R1 is chosen large enough to prevent the current flow which should pass through the discharge tube FL1 from being bypassed through the resistor R1 when the trigger thyristor SR1 is fired. Accordingly, it takes a considerable time to charge the trigger capacitor C2 after the thyristor SR1 is turned off. Hence, a second activation of the trigger circuit TC1 before the charging of the trigger capacitor C2 is complete cannot excite the discharge tube FL1 to initiate the emission of flashlight.
An electronic flash of multiple emission type is already available on the market which overcomes described disadvantages by incorporating a rapid charging controller which accomplishes a rapid charging of a trigger capacitor after each emission of flashlight so as to be ready to trigger another emission. However, the provision of such controller results in an increased size and a high price of the electronic flash, which is still incapable of achieving a reduced emission interval.
It is known that a single photograph containing a series of interrupted conditions of a continuously moving object such as the swinging process of a baseball bat or the flight of an insect is commonly referred to as stroboscopic photograph. When taking a stroboscopic photograph, one usual practice is to maintain the shutter of the camera open and to activate an electronic flash in a series of rapidly interrupted emissions. To this end, an electronic flash capable of producing interrupted emissions at a rapid rate is offered on the market and is referred to as an electronic flash of multiple emission type. However, such electronic flash of the prior art includes a plurality of main capacitors or a plurality of flash discharge tubes so that a series of flashlight emissions can be produced at a rapid rate. This results in an increased size and an increased cost of the arrangement.
To avoid such disadvantage of a conventional electronic flash of continued emission type or multiple emission type, it is desirable to provide a conventional electronic flash including a single main capacitor and a single flash discharge tube and in which the discharge tube can be activated over a continued emission interval or over a series of multiple emissions.
However, a trigger capacitor associated with a flash discharge tube is charged through a resistor in a conventional electronic flash, requiring a finite length of time to charge the capacitor. This limits the length of an interval between interrupted emissions from the discharge tube.
It will therefore be seen that it is difficult to charge a commutation capacitor rapidly, to provide a continued emission during a given time interval and at a given level, to achieve a rapid triggering operation or to produce a series of interrupted multiple emissions of flashlight at a high rate with a conventional electronic flash.